The present invention relates to a process for preparing a black liquor-phenol formaldehyde thermoset resin for use in the engineered forest products industry.
The invention also relates to the so prepared resin wherein phenol and formaldehyde are copolymerized with lignin found in a black liquor, and more preferably a sodium carbonate black liquor, which is extracted from a pulping process. Such lignin includes lignosulfonates, kraft lignin and/or sodium carbonate lignin.
The so prepared resin can be used in liquid state or it can be converted into a powder by spray drying. It is particularly useful as a binder for engineered forest products, such as wafer boards, oriented strand boards (OSB), plywood and the like.
Currently, phenol-formaldehyde (phenolic) thermoset resins are used as binders (viz. adhesives) in the engineered forest products industry. When cured, these thermoset resins bond wood chips or veneers to each other under heat and pressure. For most wood based applications, the phenolic resins that are used are Resole type resins, the pH of which is above 7 and the mole ratio of formaldehyde to phenol is greater than 1.
It is also known that black liquors, or lignins extracted from black liquors, can be copolymerized or cold mixed with phenolic resins for use in the plywood and OSB industry (see, for example, U.S. Pat. No. 4,537,941). The use of black liquors arises from the fact that black liquors contain lignin, which is a polymer containing phenolic groups with reactive sites for further polymerization. Since the black liquor is a by-product of the pulping process, the cost of the liquor is substantially lower than phenol. However, recent advancement in pressing technology has reduced from 25 seconds per millimeter to 16 seconds or less per millimeter the time required in the press to make OSB panels under elevated pressure and temperature. Because of such a reduction in time in the press cycle, the existing black liquor modified resins may not meet the CSA and ASTM standards for internal bond strength. Moreover, the existing black liquor modified resins contain small amounts of black liquor which make them economically less attractive.
It has now been discovered that by following a very specific sequence of steps, one may prepare a black liquor-phenol formaldehyde thermoset resin having improved properties as compared to those that have been prepared and used so far.
Thus, a main object of the present invention is to provide a process for preparing a black liquor-phenol formaldehyde thermoset resin, comprising the steps of:
a) mixing a black liquor with phenol and a base and letting the so obtained mixture reacts at high temperature;
b) cooling the mixture obtained in step a), then adding to it a given amount of formaldehyde and letting the so obtained mixture reacts; and
c) adding an additional amount of formaldehyde and a base to the mixture obtained in step b) in order to obtain the requested resin, such an additional amount being selected so that there is an excess of free formaldehyde in solution when the requested resin of given viscosity or molecular weight is obtained.
Preferably, in step a, the black liquor, phenol and base are mixed altogether and then the so obtained mixture is heated and kept to react.
Alternatively however, in step a):
a1) the black liquor is first mixed with phenol;
a2) the mixture obtained in sub-step a1) is heated;
a3) the base is then added to the mixture obtained in sub-step a2); and
a4) the mixture obtained in sub-step a3) is kept to react.
Preferably also, the black liquor used as starting material is a sodium carbonate black liquor. However, other kind of black liquors could alternatively be used, such as those obtained via a bisulfite process that contains lignosulfonates or via a Kraft process which uses sodium sulfide and sodium hydroxide as active alkali (Kraft lignin). However, sodium carbonate black liquor is preferred because of its reactivity and availability.
With this method, the amount of black liquor that is used, can be up to 80% by weight of the total weight of the resin (the preferred range being 40% and higher by weight of the total weight of the resin) and still meet the CSA and ASTM requirements for bond strength in Oriented Strand Board (OSB) and plywood.
Preferably also, the above method may also comprise the additional step of reacting the resin that has been obtained with urea.
Another object of the invention is the black liquor-phenol formaldehyde thermoset resin obtained by the above process.
A further object of the invention is the use of the above black liquor-phenol formaldehyde thermoset resin as a binder for engineered forest products. Preferably, prior to such a use, a potassium carbonate solution can be added to the resin so that it cures faster.
The invention and its advantages will be better understood upon reading the following non-restrictive description of it and accompanying examples.